Windows can be a death trap for birds—after all, their eyesight makes it difficult or impossible to distinguish between glass and clear flying space. Millions of birds crash into windows along their annual migratory paths and the collisions kill somewhere between 365 million to nearly one billion birds in the United States alone each year. 

Volunteers and scientists throughout the years have collected the fallen birds around the country every spring and fall to rehabilitate  injured birds and document the dead.  The bodies contain valuable scientific information, especially when they are compared over time.

[Related: How to help birds avoid crashing into your windows.]

A study published March 28 in the journal Molecular Ecology is helping scientists better understand the relationship between birds and the multiple microbes in their guts by using these unique specimens.

“In humans, the gut microbiome—the collection of bacteria, fungi, and other microbes living in our digestive tracts—is incredibly important to our general health and can even influence our behavior. But scientists are still trying to figure out how significant a role the microbiome has with birds,” co-author Heather Skeen, a biologist and research associate at Chicago’s Field Museum, said in a statement.

Different mammal species tend to have their own signature microbes living in their gut. The microbes help them digest food and fight disease, with evidence that these relationships can go back millions of years. Researchers have been finding that bird microbiomes likely play by a whole different set of rules.

“Bird gut microbiomes don’t seem to be as closely tied to host species, so we want to know what does influence them,” said Skeen. “The goal of this study was to see if bird microbiomes are consistent, or if they change over short time periods.”

Skeen focused on four common species of songbirds called thrushes, but there are dozens of species found throughout Chicago after crashing into the city’s buildings. She took samples from 747 birds over three years and included samples from the thrushes summer breeding grounds in Manitoba in Canada and the Midwestern states of Michigan and Minnesota.

To get inside of the bird bellies, she made a small incision into the abdomen to reach the bird’s intestines and squeezed out what was inside.  She then transferred bird poop from the intestines to specialized filter paper cards that preserve DNA. The genetic material was then sent away for bacteria classification. 

[Related: Puffy unicorn stickers could save millions of migrating birds each year.]

“Analyzing the bacterial DNA present in the poop allowed us to determine exactly what kinds of bacteria were present,” said Skeen. “It turns out, there were about 27,000 different types of bacteria present.”

The team looked for trends in the bacteria present across the whole sample, and found that the different bird species didn’t seem to have their own unique set of microbes—unlike mammals. Instead, time was the clearest link between the birds and the bacteria present in their microbiomes. Gut microbiomes had significant differences in the composition of the bacteria season to season and year to year.

A drawer full of thrushes in the Field Museum’s collection, killed crashing into city windows. CREDIT: Heather Skeen.

The results suggest that bird microbiomes might have more to do with their environment than the inborn, consistent relationship that is seen in most mammal species. 

Shannon Hackett, associate curator of birds at the Field Museum and a co-author of the paper, says the museum has been scooping birds killed by buildings for 40 years and that this study helps show why museum collections are valuable for research

“At the time, people were like, ‘What the hell are you doing?’ But the fact that he’s been doing this for forty years means we have a unique opportunity to study birds across fairly short periods of time. We have more than 100,000 window-killed birds at this point, it’s an incredibly rich resource,” Hackett said in a statement. “And as technology evolves and new scientists like Heather come up, we broaden what we’re able to do with these resources.”

Some ways to help birds avoid crashing into your windows include using decals and films on them that are invisible to birds while also letting light in, supporting bird-safe buildings, and turning off interior lights at night.

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